Is thinking about the possibility of a religion existing that is not from God, with no disagreement within it, and then cutting off that thought, considered blameworthy doubt or praiseworthy contemplation of the noble verse: "Had it been from [any] other than Allah, they would surely have found therein much discrepancy"?

We have previously emphasized the necessity of avoiding whispers (waswas) and turning away from them due to their harm to one's religion and worldly life. What happened to you does not constitute disbelief (kufr) or apostasy (riddah). Rather, turning away from thoughts of doubt and not indulging in them is the correct approach. The whispers that afflict a person are not held against them as long as they do not yield to them or indulge in them. The proof of this is that the hatred, detestation, and aversion to these whispers are clear signs of faith. Due to the whispers you are experiencing, merely reciting the Quran is sufficient for you, without delving into deep contemplation, to avoid whispers of doubt.